7.1 SN2 Reactions
Anatomy of a Nucleophilic Substitution Reaction
-Common leaving groups include Cl-, Br-, I- and OTs-
SN2 Reactions - Substitution Nucleophilic Bimolecular
Bimolecular – Two reactant molecules are involved in the slow step and the rate law is 2nd order.
Rate = k[substrate][nucleophile]
-The substrate is often al alkyl halide (Cl, Br, and I are good leaving groups).
-The mechanism is concerted and involves back-side attack.
-SN2 reactions result in inversion of configuration at a chiral center (Walden inversion).
Substrate Steric Effects
Effect of Beta Branching
Require a strong nucleophile – the stronger the nucleophile the faster the reaction.
Note: most strong nucleophiles are negatively charged.
-Negatively charged nucleophiles are stronger in polar aprotic solvents
Leaving Group Effects
-Generally the weaker the base the better the leaving group; the better the leaving group the faster the reaction.